In recent years, for example, the screen definition of display devices has become higher and the color reproduction ranges of display devices have become larger. The power consumption of such high performance display devices increases. For example, to solve this problem, there has been known the technique of forming a pixel of four subpixels obtained by adding a fourth subpixel which displays a fourth color to a first subpixel which displays a first primary color, a second subpixel which displays a second primary color, and a third subpixel which displays a third primary color. With this technique the fourth subpixel increases luminance. This makes it possible to decrease the luminance of a backlight. As a result, power consumption is reduced. Furthermore, the technique of controlling the luminance of a backlight according to an input image signal for reducing power consumption further is known (see, for example, Japanese Laid-open Patent Publication No. 2011-248352).